J Mater Sci(2008)43:6747-6757 DOI10.1007/10853-008-26920 STRETCHING THE ENDURANCE BOUNDARY OF COMPOSITE MATERIALS: PUSHING THE PERFORMANCE LIMIT OF COMPOSITE STRUCTURES Review of the role of the interphase in the control of composite performance on micro-and nano-length scales J.Jancar Received: 3 April 2008/ Accepted: 30 April 2008/ Published online: 16 August 2008 e Springer Science+Business Media, LLC 2008 Abstract In fiber reinforced composites(FRCs), exhib- re-define term interphase on the nano-scale. Thus, the iting heterogeneous structure at multiple length scales, the Rubinstein reptation model and a simple percolation model interphase phenomena at various length scales were shown were used to describe immobilization of chains near solid to be of pivotal importance for the control of the perfor- nano-particles and to explain the peculiarities in the vis- mance and reliability of such structures. Various models coleastic response of nano-scale"interphase. " It has also based on continuum mechanics were used to describe been shown that below 5 nm. Bernoulli-Euler mechanical response of laminates and large FRC parts, along with the proposed reptation dynamics approach, 3L effects of the macro- and meso-scale interphase on the elasticity becomes not valid and higher-order elasticity satisfactorily. At the micro-scale, the interphase is con- provide suitable means for bridging the gap in modeling sidered a 3D continuum with ascribed average properties. the transition between the mechanics of continuum matter Number of continuum mechanics models was derived over at the micro-scale and mechanics of discrete matter at the the last 50 years to describe the stress transfer between nano-scale. matrix and individual fiber with realtively good success In these models, the interphase was characterized by some average shear strength, ta, and elastic modulus, En. On the other hand, models for tranforming the properties of the Introduction micro-scale interphase around individual fiber into the mechanical response of macroscopic multifiber composite Continuum mechanics can be used to describe effects of have not been generally successfull. The anisotropy of micro-scale interphase on the stress transfer in single fiber these composite structures are the main reasons causing the composites considering the system a three phase material failure of these models. The strong thickness dependence in which the individual phases, i.e., solid inclusion, matrix of the elastic modulus of the micro-scale interphase sug- and interphase, can be characterized by some average gested the presence of its underlying sub-structure. On the properties [1-5]. Unlike at the micro-scale, extreme cau- nano-scale, the discrete molecular structure of the polymer tion has to be exercised when selecting suitable modelin has to be considered. The term interphase, originally scheme at the nano-scale when the discrete molecular roduced for continuum matter, has to be re-defined to structure of the polymer becomes obvious. One of the main include the discrete nature of the matter at this length scale. difficulties when considering nano-scale in composites is to The segmental immobilization resulting in retarded repta- determine the size of the representative volume in which tion of chains caused by interactions with solid surface the discrete nature of the composite structure has to be seems to be the primary phenomenon which can be used to taken into account [6]. Very little has been written so far on the laws governing the transition between the nano-and J. Jancar(凶 micro-length scales, especially, on the reliability of clas Institute of Materials Chemistry, Brno University of Technology, sical continuum mechanics when scaled outside their Brno, Czech Republic validity range, i.e., down to the nano-scale [7]. Thus, it e-mail: jancar@fch. vutbr cz seems desirable to perform a critical review of the current 2 Springer
STRETCHING THE ENDURANCE BOUNDARY OF COMPOSITE MATERIALS: PUSHING THE PERFORMANCE LIMIT OF COMPOSITE STRUCTURES Review of the role of the interphase in the control of composite performance on micro- and nano-length scales J. Jancar Received: 3 April 2008 / Accepted: 30 April 2008 / Published online: 16 August 2008 Springer Science+Business Media, LLC 2008 Abstract In fiber reinforced composites (FRCs), exhibiting heterogeneous structure at multiple length scales, the interphase phenomena at various length scales were shown to be of pivotal importance for the control of the performance and reliability of such structures. Various models based on continuum mechanics were used to describe effects of the macro- and meso-scale interphase on the mechanical response of laminates and large FRC parts, satisfactorilly. At the micro-scale, the interphase is considered a 3D continuum with ascribed average properties. Number of continuum mechanics models was derived over the last 50 years to describe the stress transfer between matrix and individual fiber with realtively good success. In these models, the interphase was characterized by some average shear strength, sa, and elastic modulus, Ea. On the other hand, models for tranforming the properties of the micro-scale interphase around individual fiber into the mechanical response of macroscopic multifiber composite have not been generally successfull. The anisotropy of these composite structures are the main reasons causing the failure of these models. The strong thickness dependence of the elastic modulus of the micro-scale interphase suggested the presence of its underlying sub-structure. On the nano-scale, the discrete molecular structure of the polymer has to be considered. The term interphase, originally introduced for continuum matter, has to be re-defined to include the discrete nature of the matter at this length scale. The segmental immobilization resulting in retarded reptation of chains caused by interactions with solid surface seems to be the primary phenomenon which can be used to re-define term interphase on the nano-scale. Thus, the Rubinstein reptation model and a simple percolation model were used to describe immobilization of chains near solid nano-particles and to explain the peculiarities in the viscoleastic response of nano-scale ‘‘interphase.’’ It has also been shown that below 5 nm, Bernoulli–Euler continuum elasticity becomes not valid and higher-order elasticity along with the proposed reptation dynamics approach can provide suitable means for bridging the gap in modeling the transition between the mechanics of continuum matter at the micro-scale and mechanics of discrete matter at the nano-scale. Introduction Continuum mechanics can be used to describe effects of micro-scale interphase on the stress transfer in single fiber composites considering the system a three phase material in which the individual phases, i.e., solid inclusion, matrix and interphase, can be characterized by some average properties [1–5]. Unlike at the micro-scale, extreme caution has to be exercised when selecting suitable modeling scheme at the nano-scale when the discrete molecular structure of the polymer becomes obvious. One of the main difficulties when considering nano-scale in composites is to determine the size of the representative volume in which the discrete nature of the composite structure has to be taken into account [6]. Very little has been written so far on the laws governing the transition between the nano- and micro-length scales, especially, on the reliability of classical continuum mechanics when scaled outside their validity range, i.e., down to the nano-scale [7]. Thus, it seems desirable to perform a critical review of the current J. Jancar (&) Institute of Materials Chemistry, Brno University of Technology, Brno, Czech Republic e-mail: jancar@fch.vutbr.cz 123 J Mater Sci (2008) 43:6747–6757 DOI 10.1007/s10853-008-2692-0
J Mater sci(2008)43:6747-6757 knowledge on the structure and properties of the micro- pivotal importance for the control of the reliability and and nano-scale interphases in polymer composites and the performance of multiscale FRC structures. There seems to methodologies for their modeling in order to provide be general agreement on using continuum mechanics means for bridging the gap between continuum and dis- models to account for interphase phenomena from macro- crete models useful for reliable design of future multiscale to micro-scale and the design schemes based on continuum hierarchical composite structures mechanics, variational principles or Finite Element Anal The design of multi-length-scale composite structures, ysis(FEA) have been validated [2]. The understanding of such as the fuselage of the Boeing 787(Fig. 1), represents the translation of the properties of the micro-scale inter the state-of-the-art engineering application of fiber rein- phases into the response of macroscopic FRC parts is far forced composites(FRCs). These large structures are less unambigious [9]. The greatest success have been designed from top to bottom using continuum mechanics achieved in understanding and modeling of the role of the methodologies and the transitions between the individual micro-scale interphase in the stress transfer from the matrix treated simply with down the to a single fiber structural features of the greater length scale. Such multi- attempts to transfer properties of the micro-scale interphase scale continuum mechanics modeling approach was in the performance of a multi-fiber FRC structures have demonstrated to provide reasonable means for transforming generally failed the mechanical response of polymer composites accross Over the last 20 years, substantial advances were made several length and time scales from macro- down to micro- in understanding the deformation behavior of hard tissues scale(Fig. 2)[8]. such as bones which can also be considered multiscale Since the FRCs exhibit heterogeneous structure at functionally hierarchical composite structures(Fig 3)[71 multiple length scales, the interphase phenomena are of Unlike the FRC fuselage, bone is designed bottom-up MACRO MESO mm MICRO NANO 10n micro-and nano-length scales considered in this review Fig. 1 Part of the fuselage of Boeing 787 Dreamliner can serve as an top-bottom methodology within the framework of continuum example of a large manmade multiscale composite structure. This mechanics. No functional hierarchy exists between the various length polymer composite structure has been designed using the engineering scales 2 Springer
knowledge on the structure and properties of the microand nano-scale interphases in polymer composites and the methodologies for their modeling in order to provide means for bridging the gap between continuum and discrete models useful for reliable design of future multiscale hierarchical composite structures. The design of multi-length-scale composite structures, such as the fuselage of the Boeing 787 (Fig. 1), represents the state-of-the-art engineering application of fiber reinforced composites (FRCs). These large structures are designed from top to bottom using continuum mechanics methodologies and the transitions between the individual length scales are treated simply with scalling down the structural features of the greater length scale. Such multiscale continuum mechanics modeling approach was demonstrated to provide reasonable means for transforming the mechanical response of polymer composites accross several length and time scales from macro- down to microscale (Fig. 2) [8]. Since the FRCs exhibit heterogeneous structure at multiple length scales, the interphase phenomena are of pivotal importance for the control of the reliability and performance of multiscale FRC structures. There seems to be general agreement on using continuum mechanics models to account for interphase phenomena from macroto micro-scale and the design schemes based on continuum mechanics, variational principles or Finite Element Analysis (FEA) have been validated [2]. The understanding of the translation of the properties of the micro-scale interphases into the response of macroscopic FRC parts is far less unambigious [9]. The greatest success have been achieved in understanding and modeling of the role of the micro-scale interphase in the stress transfer from the matrix to a single fiber in model composites [2–4, 8]. However, attempts to transfer properties of the micro-scale interphase in the performance of a multi-fiber FRC structures have generally failed. Over the last 20 years, substantial advances were made in understanding the deformation behavior of hard tissues such as bones which can also be considered multiscale functionally hierarchical composite structures (Fig. 3) [7]. Unlike the FRC fuselage, bone is designed bottom-up Fig. 1 Part of the fuselage of Boeing 787 Dreamliner can serve as an example of a large manmade multiscale composite structure. This polymer composite structure has been designed using the engineering top–bottom methodology within the framework of continuum mechanics. No functional hierarchy exists between the various length scales 6748 J Mater Sci (2008) 43:6747–6757 123
J Mater Sci(2008)43:6747-6757 6749 the two principal methodologies o b n designing multiscale Bridging laws omposite structures, i.e., the MACRO top-bottom engineering 101-10-1m approach observed in natural MESO opposites with the emphasi 102-103m on the role of interphase Coating MICRO henomena at various length 10-106m and time scales Bottom~up fiber-matⅸx adhesion 10-109 110∞-102m1 immobilization I 10-15s TE MESO MICRO NANO 10·101mm 100-200nm 2=4nm 123nm MACRO 0 Tissue Fibrillar Mineral particle 50 x 25*3 nm level ensuring mechanical performance of the bone and providing means natural multiscale, functionall for functional hierarchy and signaling between the various length polymer composite structure scales [assembled with help of Ref. 101 methodology with the mol starting from mineralized protein fibrils, to osteons up to a man made composites, significantly. One of the main dif- complete bone [10, 11]( Fig. 2). Despite of similar multi- ferences is in the role of the molecular interphases allowing scale structure, natural composite structures differ from the the natural composite to be hierarchical, adaptive and 2 Springer
starting from mineralized protein fibrils, to osteons up to a complete bone [10, 11] (Fig. 2). Despite of similar multiscale structure, natural composite structures differ from the man made composites, significantly. One of the main differences is in the role of the molecular interphases allowing the natural composite to be hierarchical, addaptive and Fig. 2 Schematic drawing of the two principal methodologies in designing multiscale composite structures, i.e., the top–bottom engineering approach and the bottom-up approach observed in natural composites with the emphasis on the role of interphase phenomena at various length and time scales Fig. 3 Part of the femur bone can serve as an example of a large natural multiscale, functionally hierarchical composite structure. This polymer composite structure has been designed using the bottom-up methodology with the molecularly designed discrete interphase ensuring mechanical performance of the bone and providing means for functional hierarchy and signaling between the various length scales [assembled with help of Ref. 10] J Mater Sci (2008) 43:6747–6757 6749 123
6750 J Mater sci(2008)43:6747-6757 self-repairable. In addition, the discrete molecular nature of Micro-scale interphase the"interphases"between various length scales in hard tissues result in mechanically stiff and tough natural The research of the micro-scale interphase phenomena in composites [12, 13 composite materials has attracted considerable attention of Q. In this paper, the interphase phenomena in the manmade both the scientific and engineering communities over the and natural polymer matrix composite structures at the last 50 years. Good succes has been achieved in describing micro-and nano-scales are briefly reviewed. The bridging the role of the interphase in stress transfer from the matrix laws for transformation of properties of the discrete matter at to the fiber using model single fiber composites(Fig. 4) the nano-scale to the continuum matter at the micro-scale From the simple Kelly-Tyson model, to the various lap based on the combination of gradient strain elasticity and shear models, to the numerical F E.A. models, the approach reptation dynamics of a chain above Tg, will also be outlined. based on the continuum mechanics has been employed interphase considering only the (a) Fig 4(a) Visualizing the Bulk polymer Interphase Stress transfer micro-scale. Interphase is a ontinuum layer with a gradient 50 um Diffuse interfa Fiber/inclusion in its structure. The main role of the micro-scale interphase is to provide stable and effective means for stress transfer between inclusions and polyn matrix even under adverse 10 um Sharp interface structure of a micro-composite onside i Property gradient discrete structure of the matrix Filler adhesion and inclusions becomes evident uter 0000050.10 015020 Normalized Distance Micro-scale Nano-scale Polymer matriⅸx Micro-scale interphase ber/inclusion Nano-scale 2 Springer
self-repairable. In addition, the discrete molecular nature of the ‘‘interphases’’ between various length scales in hard tissues result in mechanically stiff and tough natural composites [12, 13]. In this paper, the interphase phenomena in the manmade and natural polymer matrix composite structures at the micro- and nano-scales are briefly reviewed. The bridging laws for transformation of properties of the discrete matter at the nano-scale to the continuum matter at the micro-scale, based on the combination of gradient strain elasticity and reptation dynamics of a chain above Tg, will also be outlined. Micro-scale interphase The research of the micro-scale interphase phenomena in composite materials has attracted considerable attention of both the scientific and engineering communities over the last 50 years. Good succes has been achieved in describing the role of the interphase in stress transfer from the matrix to the fiber using model single fiber composites (Fig. 4). From the simple Kelly-Tyson model, to the various lap shear models, to the numerical F.E.A. models, the approach based on the continuum mechanics has been employed Fig. 4 (a) Visualizing the interphase considering only the micro-scale. Interphase is a continuum layer with a gradient properties reflecting variations in its structure. The main role of the micro-scale interphase is to provide stable and effective means for stress transfer between inclusions and polymer matrix even under adverse conditions. (b) Visualizing the structure of a micro-composite considering also the nano-scale structural features when the discrete structure of the matrix and inclusions becomes evident 6750 J Mater Sci (2008) 43:6747–6757 123
J Mater Sci(2008)43:6747-6757 [14]. Even though the molecular structure of the interphase temperatures of engineering thermoplastics ranging from has been anticipated in many papers, with some exceptions 230 to 350C can exceed the thermal stability of the [15-17]. the main effort has been devoted to the relation- commonly utilized organosilanes ship between the type, thickness and deposition conditions Thickness dependence of the elastic modulus of thin of the fiber coating and the average shear strength of the polycarbonate (PC) layers deposited on a flat E-glass interphase, ta, measured in a simple test employing model substrate was measured over the thickness interval ranging single fiber composite [18]. from 10 to 30 nm [ 9, 21, 22). In all cases investigated Mechanical properties and environmental stability of elastic moduli of the deposited layers, Ei, decreased composites are strongly dependent upon the stability of the constant bulk value for layers thicker than 5 X 10'mr9 both fiber reinforced and particulate filled thermoplastic monotonically with increasing layer thickness reaching interfacial region between the matrix and fibers, especially(Fig. 4). Thermally annealed PC and SiCl4 grafted oligo- when exposed to moist environment. This is of particular PC interphases, exhibited higher elastic moduli than the as importance in glass fiber reinforced thermoplastic compos- received solution deposited PC interphase. No effect of ites since the glass fibers are highly hygroscopic and the thermal annealing on elastic modulus of strongly bonded bond between the fibers and the thermoplastic matrix is oligo-PC interphase was observed. It has been shown that usually weak. Hence, the tailoring of well-bonded, durable the shear strength of the interface, Ta, measured in a sin- interphases between the thermoplastic matrix and glass glefiber fragmentation test exhibited strong dependence on reinforcement has become a critical concern. The use of the interphase Ei. coupling agents, chemically reactive with both matrix and Similarly to the PC interphases, elastic moduli of the reinforcement, and/or chemical modification of the surfaces deposited silane layers decreased monotonically with of one or both constituents have been the most successful increasing layer thickness reaching a bulk value for layers means of providing reasonably well controlled bond thicker than 10 nm [21, 22]. Reactive chlorine containing between matrix and the encapsulated reinforcement [ 19] silane formed always stiffer layers compared to its alkoxy From the published data, it seems clear that a mono- analogues most probably due to stronger interaction molecular interphase layer with engineered molecular between the chlorine and glass surface and, most probably structure specific for the desired combination of resin and due to less defective network structure (Fig. 5). This reinforcement should result in the most favorable mix of hypothesis was further supported by the observed strong properties in thermoplastic matrix composites. Reactive effect of deposition technique, controlling the layer su- end-capped polymers capable of chemically reacting with permolecular structure, on the layer elastic modulus the fiber surface or various methods of grafting matrix Solution deposition technique yielded always layers with molecules onto reinforcement surface are the most prom- lower elastic modulus compared to the layers formed by rf- ising candidates for further investigations [17, 20]. plasma or rf-plasma enhanced CVd deposition of the same Thickness of the interphase can be controlled via modifi- substance. A qualitative explanation of the observe cation of the molecular weight and chain stiffness of the behavior was provided assuming formation of strongly constituent molecules, its mechanical properties can be immobilized layer of constant thickness, t;, and elastic varied by selecting the backbone chain constitution and modulus, Ei, near the bonded interface. Strength of the configuration, and its surface free energy can also be interfacial bond and network density of the polysiloxane controlled by the chain constitution and by the polarity of interphase were proposed to be the factors determining the end-groups. Elastic properties of these layers are con- and Ei for the given external conditions. Experimental data trolled by the attraction forces at the interface as well as the showed that the contribution of this strongly immobilized conformation entropy of the chains forming the layer. layer started to play an important role for interphase Organofunctional silanes are so far the most widely used thickness below 10 nm. This"inner"layer has been coupling agents for improvement of the interfacial adhe- covered with weaker "outer"layer with more defective sion in glass reinforced materials [19]. Upon application of network structure. The thickness of the"outer"layer was a silane from either dilute solution or the vapor phase, a dependent on the concentration of the silane solution it was highly crosslinked multilayer siloxane "interphase"is deposited from. The difference in E; between the outer presumably formed with thickness ranging from 1.5 to inner interphase layer was increasing with strengthening 500 nm. Unlike in thermosetting matrices with extensive the layer-surface interaction interpenetration between organosilane layer and the matrix In order to enhance the performance and reliability of monomer,long chain molecules do not interpenetrate the the FRC structures at the macro-scale, the results obtained organosilane layers significantly. On the other hand, for the micro-scale interphase can be used to control the immobilization phenomena are of a greater importance in stress transfer between the matrix and the reinforcement hermoplastic matrix composites. Moreover, processing Stiff interphases provide very efficient stress transfer, less 2 Springer
[14]. Even though the molecular structure of the interphase has been anticipated in many papers, with some exceptions [15–17], the main effort has been devoted to the relationship between the type, thickness and deposition conditions of the fiber coating and the average shear strength of the interphase, sa, measured in a simple test employing model single fiber composite [18]. Mechanical properties and environmental stability of both fiber reinforced and particulate filled thermoplastic composites are strongly dependent upon the stability of the interfacial region between the matrix and fibers, especially when exposed to moist environment. This is of particular importance in glass fiber reinforced thermoplastic composites since the glass fibers are highly hygroscopic and the bond between the fibers and the thermoplastic matrix is usually weak. Hence, the tailoring of well-bonded, durable interphases between the thermoplastic matrix and glass reinforcement has become a critical concern. The use of coupling agents, chemically reactive with both matrix and reinforcement, and/or chemical modification of the surfaces of one or both constituents have been the most successful means of providing reasonably well controlled bond between matrix and the encapsulated reinforcement [19]. From the published data, it seems clear that a monomolecular interphase layer with engineered molecular structure specific for the desired combination of resin and reinforcement should result in the most favorable mix of properties in thermoplastic matrix composites. Reactive end-capped polymers capable of chemically reacting with the fiber surface or various methods of grafting matrix molecules onto reinforcement surface are the most promising candidates for further investigations [17, 20]. Thickness of the interphase can be controlled via modifi- cation of the molecular weight and chain stiffness of the constituent molecules, its mechanical properties can be varied by selecting the backbone chain constitution and configuration, and its surface free energy can also be controlled by the chain constitution and by the polarity of the end-groups. Elastic properties of these layers are controlled by the attraction forces at the interface as well as the conformation entropy of the chains forming the layer. Organofunctional silanes are so far the most widely used coupling agents for improvement of the interfacial adhesion in glass reinforced materials [19]. Upon application of a silane from either dilute solution or the vapor phase, a highly crosslinked multilayer siloxane ‘‘interphase’’ is presumably formed with thickness ranging from 1.5 to 500 nm. Unlike in thermosetting matrices with extensive interpenetration between organosilane layer and the matrix monomer, long chain molecules do not interpenetrate the organosilane layers significantly. On the other hand, immobilization phenomena are of a greater importance in thermoplastic matrix composites. Moreover, processing temperatures of engineering thermoplastics ranging from 230 to 350 C can exceed the thermal stability of the commonly utilized organosilanes. Thickness dependence of the elastic modulus of thin polycarbonate (PC) layers deposited on a flat E-glass substrate was measured over the thickness interval ranging from 106 to 30 nm [9, 21, 22]. In all cases investigated, elastic moduli of the deposited layers, Ei, decreased monotonically with increasing layer thickness reaching a constant bulk value for layers thicker than 5 9 105 mm (Fig. 4). Thermally annealed PC and SiCl4 grafted oligoPC interphases, exhibited higher elastic moduli than the as received solution deposited PC interphase. No effect of thermal annealing on elastic modulus of strongly bonded oligo-PC interphase was observed. It has been shown that the shear strength of the interface, sa, measured in a singlefiber fragmentation test exhibited strong dependence on the interphase Ei. Similarly to the PC interphases, elastic moduli of the deposited silane layers decreased monotonically with increasing layer thickness reaching a bulk value for layers thicker than 105 nm [21, 22]. Reactive chlorine containing silane formed always stiffer layers compared to its alkoxyanalogues most probably due to stronger interaction between the chlorine and glass surface and, most probably, due to less defective network structure (Fig. 5). This hypothesis was further supported by the observed strong effect of deposition technique, controlling the layer supermolecular structure, on the layer elastic modulus. Solution deposition technique yielded always layers with lower elastic modulus compared to the layers formed by rfplasma or rf-plasma enhanced CVD deposition of the same substance. A qualitative explanation of the observed behavior was provided assuming formation of strongly immobilized layer of constant thickness, ti, and elastic modulus, Ei, near the bonded interface. Strength of the interfacial bond and network density of the polysiloxane interphase were proposed to be the factors determining ti and Ei for the given external conditions. Experimental data showed that the contribution of this strongly immobilized layer started to play an important role for interphase thickness below 103 nm. This ‘‘inner’’ layer has been covered with weaker ‘‘outer’’ layer with more defective network structure. The thickness of the ‘‘outer’’ layer was dependent on the concentration of the silane solution it was deposited from. The difference in Ei between the outer and inner interphase layer was increasing with strengthening the layer-surface interaction. In order to enhance the performance and reliability of the FRC structures at the macro-scale, the results obtained for the micro-scale interphase can be used to control the stress transfer between the matrix and the reinforcement. Stiff interphases provide very efficient stress transfer, less J Mater Sci (2008) 43:6747–6757 6751 123